The present invention relates to an optical semiconductor device and a method for fabricating the same, more specifically an optical semiconductor device including an active layer of quantum dot layers stacked one on another and a method for fabricating the optical semiconductor device.
As the optical fiber communication is more sped up and has larger capacities, the all-optical signal processing, such as 1R (regeneration), 2R (regeneration, reshaping) and 3R (regeneration, reshaping, retiming), which does not convert optical signals to electrical signals at the transit points of a network is required. Wavelength switches which are operative at ultra-high speed are required so as to realize free connections among sub-networks without the collision between wavelength bands (channels) used in a photonic network.
As a device for the all-optical signal processing and the wavelength switch, SOAs (Semiconductor Optical Amplifiers) are noted, and various studies are being made. SOAs are used not only as a device for amplifying light, but also as a wavelength switching device utilizing the nonlinear effect, i.e., an XGM (Cross Gain Modulation) device and an FWM (Four-wave Mixing) device.
Presently, SOAs having the bulk active layer or the quantum well active layer are produced but are limited in the high bit-rate all-optical signal processing ability. One cause for this is the pattern effect in the uses as amplifying devices and the wavelength switching devices using XGM. That is, when SOAs having the bulk active layer or the quantum well active layer are used near the gain saturation region of outputs, the gain recovery cannot follow bit rates, and the output pulse waveforms are disturbed. The wavelength conversion by FWMs does not have sufficient conversion efficiency, and the wavelength becomes unsymmetrical between positive detuning and negative detuning. A converted wavelength range where the S/N ratio is sufficient is accordingly limited. The SOA having quantum well active layers is described in, e.g., Reference 1 (Japanese published unexamined patent application No. 2003-017812).
To solve this problem it is proposed to use quantum dots in the active layer, and active studies for realizing the device are being made. Quantum dots are so quick to recover gains in comparison with bulks and quantum wells that the pattern effect never takes place even when used in a gain saturation region. The wavelength conversion by FWM using quantum dot SOAs may not depend on conversion directions. What has been described above has been empirically proved. SOAs using quantum dots are described in, e.g., Reference 2 (Tomoyuki Akiyama et al., “Pattern-effect-free semiconductor optical amplifier achieved using quantum dots”, Electronics Letters, Sep. 12, 2002, Vol. 38, No. 19, pp. 1139-1140) and Reference 3 (Tomoyuki Akiyama et al., “Symmetric highly efficient (˜0 dB) wavelength conversion based on four-wave mixing in quantum dot optical amplifiers”, IEEE Photonics Technology Letters, Aug. 8, 2002, Vol. 14, No. 8, pp. 1139-1141).
Then, the conventional optical semiconductor device using quantum dots will be explained with reference to FIG. 9. FIG. 9 is a diagrammatic sectional view of the conventional optical semiconductor device, which shows the structure thereof.
An SCH (Separate Confinement Hetero-structure) layer 102 is formed on a semiconductor substrate 100. An active layer 104 formed of a stack of quantum dot layers 106 and barrier layers 108 repeatedly stacked the latter on the former is formed on the SCH layer 102. An SCH layer 110 is formed on the active layer 104. A clad layer 112 and a contact layer 114 are formed on the SCH layer 110. The clad layer 112 and the contact layer 114 are patterned in a mesa-configuration. Ap-side electrode 118 is formed on the contact layer 114, which is on the top of the mesa, with an insulating film 116 formed therebetween. An n-side electrode 120 is formed on the backside of the substrate 100.
Thus, the optical semiconductor device having an active layer having a stack of a plurality of quantum dot layers is constituted.